Field of the Invention and Description of the Related Art
The present invention relates to a circuit for processing image signals read out of a pick-up apparatus having at least first and second solid state image sensing devices adopting a so-called spatial pixel shift, in which a number of light receiving elements of the first solid state image sensing device are spatially shifted with respect to light receiving elements of the second solid state image sensing device in a main scanning direction by a distance which is substantially equal to a half of a pitch of the arrangement of light receiving elements in the main scanning direction.
In the solid state image sensing device, a number of light receiving elements are spatially arranged independently from each other, so that the spatial sampling is performed. According to the Nyquist's sampling theorem, a spatial frequency of an image of an object which can be reproduced by a single solid state image sensing device is limited to a frequency range up to f.sub.c /2, wherein f.sub.c is a horizontal clock frequency for reading signal charges stored in the solid state image sensing device. If a frequency range higher than f.sub.c /2 is to be obtained, there is produced a noise signal due to the fact that higher frequency components are folded back toward a lower frequency band. In a three-plate-type color television camera, in order to attain a higher resolution, a solid state image sensing device for obtaining a green color signal is spatially arranged with respect to the remaining two solid state image sensing devices for producing red and blue color signals such that light receiving elements of the green image sensing device are shifted with respect to light receiving elements of the red and blue image sensing devices in the horizontal scanning direction by a distant which is substantially equal to a half of a pitch of the arrangement of the elements in the horizontal scanning direction. Such a method is generally called a spatial pixel shift.
FIGS. 1A to 1C show a known solid state image pick-up apparatus in which the above mentioned spatial pixel shift method is adopted. Light from an object is made incident upon a color separation optical system 2 by means of an objective lens 1 and is divided into red, green and blue light, which are then made incident upon respective solid state image sensing devices 3R, 3G and 3B. As illustrated in FIG. 1B, pixels of the green image sensing device 3G are shifted with respect to those of the red and blue image sensing devices 3R and 3B in the main scanning direction by a half of a pitch P at which the pixels are arranged in the main scanning direction. When such a spatial pixel shift method is utilized, the image of the object is spatially sampled such that the red and blue pixels are positioned between successive green pixels as depicted in FIG. 1C. Therefore, when a brightness signal is produced by mixing the red, green and blue color signals, the number of pixels is apparently increased, and thus the resolution of the brightness signal becomes higher and the false signal can be decreased.
As explained above, in the spatial pixel shift method, the light receiving elements of the green image sensing device 3G are spatially shifted with respect to those of the red and blue image sensing devices 3R and 3B by P/2 in the main scanning direction. Therefore, prior to forming the brightness signal by mixing the color signals generated by these color image sensing devices, it is necessary to delay the green color signal with respect to the red and blue color signals by a time period corresponding to P/2, i.e. a half of a period of the signal reading clock in signal processors 4R, 4G and 4B.
Now a method of effecting the above delay will be explained for a known correlation double sampling method which has been widely used to remove reset noise and amp-noise from an output signal generated by CCD (Charge Coupled Device) which has been commonly utilized as the solid state image sensing device FIG. 2 shows the construction of the correlation double sampling circuit An input color signal read out of a solid state image sensing device is parallelly supplied to first and second sample and hold circuits 5 and 6. Output signals of the first and second sample and hold circuits 5 and 6 are then supplied to a differential amplifier 7.
FIG. 3A depicts the input color signal. To the first sample and hold circuit 5 is supplied a first sampling signal SH-1 shown in FIG. 3B and the input color signal is sampled at a feed-through portion thereof. To the second sample and hold circuit 6 is supplied a second sampling signal SH-2 illustrated in FIG. 3C and the input color signal is sampled at a signal portion thereof. In this manner, the input color signal is sampled and held by the first and second sampling signals SH-1 and SH-2 at suitable timings and then a difference between these sample values is derived by the differential amplifier 7. In this manner, it is possible to derive the output color signal having high S/N without being influenced by reset noise and amp-noise.
FIG. 4 shows the construction of the known correlation double sampling circuit. The green color signal read out of the green image sensing device 11G is amplified by a buffer amplifier 12G and is then parallelly supplied to first and second switches 13G and 14G. The first switch 13G is operated by the first sampling signal SH-1 shown in FIG. 3B and the green color signal generated from the green image sensing device 11G is sampled at the feed-through portion. The sampled signal is held in a first hold circuit 15G. The second switch 14G is driven by the second sampling signal SH-2 illustrated in FIG. 3C and the green color signal is sampled at the signal portion to derive a sample value which is stored in a second hold circuit 16G. The sample value held in the first hold circuit 15G is transferred via buffer amplifier 17G and third switch 18G to a third hold circuit 19G, and is further supplied via a buffer amplifier 20G to one input of a differential amplifier 21G. The sample value stored in the second hold circuit 16G is supplied via a buffer amplifier 22G to the other input of the differential amplifier 21G. The second and third switches 14G and 18G are driven by the second sampling signal SH-2. Therefore, the green color signal read out of the green image sensing device 11G is sampled at the signal portion to derive a sample value and the thus derived sample value is stored in the second hold circuit 16G. At the same time, the sample value at the feed-through portion is stored in the third hold circuit 19G. In this manner, the sample values representing the signal levels of the green color signal sampled at the feed-through portion and signal portion are simultaneously supplied to the differential amplifier 21G, and thus the differential amplifier produces an output green signal.
The red and blue color signals read out of the red and blue image sensing devices are processed in the entirely same manner, so that here only the processing circuit for the red color signal is shown in FIG. 4. In the red color signal processing circuit, portions similar to those of the green color signal processing circuit are denoted by the same reference numerals with R instead of G. As explained above, the light receiving elements of the red image sensing device 11R are spatially shifted with respect to those of the green image sensing device 11G in the main scanning direction by P/2, so that sampling timings for the red color signal have to be changed with respect to those for the green color signal. To this end, there are arranged switches 23R, 24R, hold circuits 25R, 26R and buffer amplifiers 27R, 28R. The switches 23R and 24R are driven by a third sampling signal SH-3 shown in FIG. 3D. The third sampling signal SH-3 is shifted with respect to the second sampling signal SH-2 by a half of the period of the clock pulses for reading the solid state image sensing devices. Therefore, the sample values stored in the hold circuits 16R and 19R are transferred into the hold circuits 25R and 26R, respectively at the timing of the third sampling signal SH-3, and are then supplied to the differential amplifier 21R by means of the buffer amplifiers 28R and 27R, respectively. In this manner, from the differential amplifier 21R there is derived an output red color signal which has been delayed with respect to the green color signal by a half of the clock period. The thus processed color signals are then supplied to succeeding stages and there are produced the brightness signal and color difference signals in a usual manner.
In the above mentioned known signal processing system, the frequency response of the brightness signal is decreased in proportion to the increase in the spatial frequency of the object, and when the spatial frequency becomes equal to the clock frequency for the solid state image sensing devices, the frequency response becomes zero. This will be further explained in detail with reference to FIGS. 5A to 5F. FIG. 5A illustrates the object having a regular repetition of bright portions and dark portions, and FIGS. 5B and 5C show the second and third sampling signals SH-2 and SH-3. When such an object is picked-up and the read out signal is processed by the known correlation double sampling circuit, each of the green and red signals lasts for a time interval between successive sampling points as shown in FIGS. 5D and 5E. Then, the frequency response of the brightness signal obtained by mixing the green, red and blue color signals with each other at a predetermined ratio becomes zero as shown in FIG. 5F.